Healthcare and Medicine Reference
In-Depth Information
The ground substance is a watery gel composed of
mucopolysaccharides or glycosaminoglycans such as
hyaluronic acid, chondroitin sulfate, keratin sulfate, and
heparin sulfate. These fern-like colloids, which are part
of the environment of nearly every living cell, bind
water in such a way as to allow the easy distribution of
metabolites (at least, when the colloids are sufficiently
hydrated), and form part of the immune system barrier,
being very resistant to the spread of bacteria. Produced
by the fibroblasts and mast cells, this proteoglycan forms
a continuous but highly variable 'glue' to help the tril-
lions of tiny droplets of cells both hold together and yet
be free to exchange the myriad substances necessary for
living. In an active area of the body, the ground sub-
stance changes its state constantly to meet local needs;
in a 'held' or 'still' area of the body, it tends to dehydrate
to become more viscous, more gel-like, and to become
a repository for metabolites and toxins. The synovial
fluid in the joints and the aqueous humor of the eye are
examples where ground substance can be seen in large
quantities, but smaller amounts of it are distributed
through every soft tissue.
How to build a body
To stand and walk, a human requires diverse and
complex building materials. As a thought experiment,
imagine that we were going to build a body out of
things that could be bought in a local hardware store or
builder's supply. We will imagine that we have already
engaged Apple® (of course) to build the computer to run
it, and that we have already obtained little servo-motors
for the muscles, but what would we need to buy to build
an actual working model of the body's structure? Put
less archly, what kind of structural materials can con-
nective tissue cells fashion?
You might suggest wood, PVC pipe, or ceramic for
the bones, silicon or plastic of some sort for the cartilage,
string, rope, and wire of all kinds, hinges, rubber tubing,
cotton wool to pack the empty places, cling-wrap and
plastic bags to seal things off, oil and grease to lubricate
moving surfaces, glass for the lens of the eye, cloth and
plastic sacks, filters and sponges of various kinds. And
where would we be without Velcro® and duct tape?
The list could go on, but the point is made: connective
tissue cells make biological correlates of all these materi-
als and more, by playing creatively with cell function
and the two elements of the ECM - the tough fiber
matrix and the viscous ground substance. The fibers and
ground substance, as we shall see, actually form a con-
tinuous spectrum of building materials, but the distinc-
tion between the two (non-water-soluble collagen fiber
and hydrophilic proteoglycans) is commonly used. The
ECM, as we will learn in the section on tensegrity, is
actually continuous with the intracellular matrix as
well, but for now, once again the distinction between
what is outside the cell and what is inside is useful. 1 6
Table 1.1 summarizes the way in which the cells alter
the fibers and the interfibrillar elements of connective
Fig. 1.8 This photomicrograph shows very clearly the fibroblasts
extruding tropocollagen, which combines into the three-strand
collagen molecule along the bottom. There are also bendy yellow
elastin fibers, and the much smaller reticulin fibers (© Prof. P.
Motta/Science Photo Library. Reproduced with kind permission.)
Connective tissue cells create a stunning variety of building materials by altering a limited variety of fibers and interfibrillar elements. The
table shows only the major types of structural connective tissues, from the most solid to the most fluid.
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